In a typical reciprocating piston pump such as that shown in FIGS. 1 and 2, there is a seal that separates the high pressure working fluid from the atmosphere surrounding the pump. Even in perfect condition, these seals can weep a small amount of fluid each cycle and this can be detrimental to seal life. The fluid can solidify or crystallize and be pulled back into the seal, shortening the seal life. Historically one way to deal with this small amount of leakage has been to use a flexible bellows seal, which creates a pump without an exposed sliding seal. In such designs, the inlet of the pump is routed past the high pressure seal and the resulting low pressure inlet chamber is sealed by the bellows which creates an air tight non-sliding seal.
As the pump reciprocates, the bellows changes volume and thus the side of the bellows not contacting the working fluid is usually vented to the atmosphere to prevent pressure from building up. The problem with this basic layout is that when the bellows fails as a result of various conditions including fatigue, over-pressurization, or excessive speed, it can cause a substantial exterior leak. This is especially true if there is a positive pressure in the inlet chamber. This does not happen with a normal sliding seal which usually fails in a very slow predictable fashion, versus a sudden rupture.